A biaxially oriented polyester film typified by a polyethylene terephthalate film is used for various uses, especially for a magnetic recording medium, a condenser or the like due to its excellent physical and chemical properties.
In the biaxially oriented polyester film, lubricity and abrasion resistance are crucial factors which greatly influence the handling properties at the film manufacturing step and the processing step, and which further greatly influence the quality of the product. When these properties are poor, the biaxially oriented polyester film has a problem, for example, in the application process of a magnetic layer on the surface of the film for using this as a magnetic tape, the friction between the coating roll and the surface of the film is strong, and the abrasion of the film is heavy, and the film is apt to undergo wrinkle and scratch on its surfaces. When the film is used as a base film for a tape of a VTR or a datum cartridge, during the process of taking out from or winding up into a cassette or the like, friction is generated between the film and many guiding parts, a reproducing head or the like, scratching and distortion occur, further white dusts are formed by the abrasion of the surface of a base film, and so forth. These often become the causes of the occurrence of a so-called drop-out.
There have been many studies concerning these problems, especially a method for adding silicone resin particles (for example, Japanese Unexamined Patent Publication No. 62-172031) has a large improving effect, and is expected to have great potentialities as a technology for solving these problems.
However, even in the film produced by these methods, there have been indicated new problems such as the increase of the amount of white dusts generated under severe conditions, for example, high speed treatment for getting high productivity in the processes of application of magnetic layer and calendering in the production of a videotape, high speed dubbing of a soft tape, repeated running and rewinding, and so forth, which are recently prevailing.
Moreover, conventionally used silicone resin fine particles contain coarse particles and aggregated particles, and the improvement of silicone resin fine particles has been desired because, for example, coarse protrusions, which is called flyspecks, are formed to cause a drop-out when said silicone resin fine particles are applied to a base film for a magnetic recording tape which must have further higher electromagnetic conversion characteristics.
On the other hand, considering condenser use, downsizing and duty enlargement of a condenser become important quality conditions in condenser industry with the advance of recent requirement for downsizing an electric or electronic circuit. In such circumstances, in order to supply the film which is a dielectric material and becomes the base film of a film condenser, the technology to produce thin film is being developed. In the film condenser, why the decreasing of the thickness of the film as a dielectric material is sought is (a) an electrostatic capacity of a condenser is proportional to a permittivity electrode area of a dielectric material, and (b) the electrostatic capacity is inversely proportional to the thickness of the film, in other words, the dielectric capacity per unit volume of a dielectric material is inversely proportional to the square of the thickness of a film and proportional to the permittivity. Thereby, it becomes inevitable to decrease the thickness of a film for downsizing or duty enlargement of a condenser as far as a dielectric material having a same permittivity is used.
It is important to decrease the thickness of a film, but the simple decreasing of the thickness in a conventional stretched-film has the problems shown below. For example, as the thickness of the film decreases, handling properties in the steps of the evaporation of an electrode on the film, the slitting of the evaporated film, the winding of raw tape and so on become poor.
These handling properties are related to the lubricity of a film, and in order to improve the lubricity, a method imparting fine concavo-convex deformations to the surfaces of the film is generally known in a thermoplastic resin film. As examples of such a method, a technology (an external particle-addition method) in which inert particles are added during or after the polymerization of the thermoplastic polymer which is a raw material of the film and a technology (an internal particle deposition method) in which a part or the whole of the catalyst or the like which is used for polymerization of the thermoplastic polymer is deposited into the polymer during the reaction processes are known.
However, in a method for producing an ultra-thin film, when a polymer is subjected to a process for producing a thinner film without increasing the concentration of inert fine particles, the number of the inert fine particles per unit area decreases, the distances between fine particles on the surface increase, the film surface becomes flat, and as a result, the lubricity tends to lower. In order to make up for the lowering of fluidity which is accompanied by the decrease of the thickness of the film, it is required to increase the concentration of the inert fine particles added or to increase the diameters of the particles as the thickness of the film decreases.
In this case, owing to poor affinity between the inert fine particles and the thermoplastic polymer, a large number of voids are formed on interfaces, i.e., around the inert particles, especially during melt extrusion or drawing at a high draft ratio, and as the result of the formation of said voids, not only mechanical properties (for example, breaking strength and breaking elongation) of the obtained film are extremely deteriorated or a breakdown voltage extremely lowers, but also ruptures are apt to occur during the production of the film, and resultingly there have been problems of the fall of productivity and the lack of stability of production conditions.